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SubjectRe: [PATCH] Input: tca8418_keypad: hide gcc-4.9 -Wmaybe-uninitialized warning
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On Monday, October 24, 2016 4:45:13 PM CEST Dmitry Torokhov wrote:
> > diff --git a/drivers/input/keyboard/tca8418_keypad.c b/drivers/input/keyboard/tca8418_keypad.c
> > index 9002298698fc..3048ef3e3e16 100644
> > --- a/drivers/input/keyboard/tca8418_keypad.c
> > +++ b/drivers/input/keyboard/tca8418_keypad.c
> > @@ -164,11 +164,18 @@ static void tca8418_read_keypad(struct tca8418_keypad *keypad_data)
> > int error, col, row;
> > u8 reg, state, code;
> >
> > - /* Initial read of the key event FIFO */
> > - error = tca8418_read_byte(keypad_data, REG_KEY_EVENT_A, &reg);
> > + do {
> > + error = tca8418_read_byte(keypad_data, REG_KEY_EVENT_A, &reg);
> > + if (error < 0) {
> > + dev_err(&keypad_data->client->dev,
> > + "unable to read REG_KEY_EVENT_A\n");
> > + break;
> > + }
> > +
> > + /* Assume that key code 0 signifies empty FIFO */
> > + if (reg <= 0)
> > + break;
>
> I am unconvinced that this rearrangement fixes the issue for all older
> GCCs. Can we simply do:
>
> u8 uninitialized_var(reg);
>
> and be done with it?

Yes, that would work. However:

- avoiding the fake intialization tends to produce better object
code, as gcc actually knows what's going on
- Linus doesn't like uninitialized_var() and would rather see it
removed from the kernel
- llvm produces warnings for uninitialized_var()

I have checked gcc-4.6/4.7/4.8/4.9/5.x/6.x, and only gcc-4.9
produces the warning. 4.9 changed the detection for uninitialized
variables significantly, which generally caused fewer false
positives but unfortunately introduced a couple of new ones
like this.

Arnd

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